Voltage pulse train generator, application to the control of an ultrasound piezoelectric injector

ABSTRACT

A voltage pulse train generator which may find application to control of an ultrasound piezoelectric injector, and including a voltage source providing a DC initial voltage, a DC/DC converter supplied with the initial voltage and configured to charge a capacitor according to an intermediate DC voltage greater than the initial voltage, a DC/AC converter operating by switching, by alternating active phases and inactive phases, which is configured to transform the intermediate voltage from the capacitor into a final voltage pulse train, and a control unit provided for driving the converters. The DC/DC converter is configured to operate to charge the capacitor at a same time as the DC/AC converter, at most during the inactive phases of the switching of the DC/AC converter.

The present invention relates to a voltage pulse train generator and itsapplication to the control of an ultrasound piezoelectric injector.

An ultrasound piezoelectric injector is a fuel injector that can be usedin an internal combustion engine. It operates according to a principlethat is described, for example, in the patent FR 99/14548. With respectto its control, it takes the form of an electric dipole and requires anAC voltage of high amplitude to be imposed across its two terminals inorder to produce a mechanical resonance and thus obtain an injection offuel throughout the time of application of said AC voltage.

Such an ultrasound piezoelectric generator requires control electronics,such as those described in FR 04/13277, capable of controlling it bygenerating voltage pulse trains, a pulse train beginning with the startof the injection phase of the combustion engine for said injector andending with the end of said injection phase.

Referring to FIG. 1, this shows one embodiment of such a control device12. This control device 12 comprises a voltage pulse train generator 11and an injector selector 14 for switching the generated pulse traintoward an injector 13 determined by choice. The voltage pulse traingenerator 11 comprises a voltage source 1, a direct current/directcurrent converter, denoted DC/DC converter 3, a directcurrent/alternating current converter, denoted DC/AC converter 6, and acontrol unit 10. The voltage source 1 supplies an initial DC voltage 2to the DC/DC converter 3. Said DC/DC converter 3 is able to charge acapacitor 4, placed at its output, to an intermediate DC voltage 5greater than the initial voltage 2. As an indication, if the initialvoltage 2 obtained from the vehicle battery/network is approximatelyequal to 12 V, the intermediate voltage 5 at the capacitor 4 may bearound 250 V. The DC/AC converter 6 is powered from the intermediatevoltage 5. The latter, which operates according to the switchingprinciple by alternating active phases 7 and inactive phases 8, is ableto transform the intermediate voltage 5 into a final AC voltage 9 of anamplitude that is further increased. If the intermediate voltage 5 isaround 250 V, the final voltage or amplitude of the final voltage pulsetrain 9 can reach between 1.2 and 1.8 kV. An injector 13 injects when itreceives said final AC voltage 9. The latter voltage is produced whenthe injector 13 is in the injection phase. It is then interrupted, forexample by stopping the drawing of the switching of the DC/AC converter6 between two injection phases. The final voltage thus takes the form ofa succession of voltage pulse trains, separated by zero voltage phases.A control unit 10 drives the converters 3, 6, and notably controls theswitching.

The design principle of such a voltage pulse train generator 11, withtwo cascaded converters 3, 6, makes the simultaneous operation of theDC/DC converter 3 and the DC/AC converter 6 complicated for reasons ofstability. Either the DC/DC converter 3 operates and charges thecapacitor 4, or the DC/AC converter 6 operates and thus causes saidcapacitor 4 to discharge. This is not prejudicial to the injectorcontrol application, since the final AC signal or final voltage pulsetrain 9 does not need to be produced continuously, and the non-injectionphases in which the DC/AC converter 6 does not operate can be exploitedto operate the DC/DC converter 3, in order to recharge the capacitor 4.

However, such a usage principle that alternates two steps, charging ofthe capacitor 4 outside the injection phase, and discharging during theinjection phase, presents a number of drawbacks. A first drawback isassociated with the duration of the injection phase which can be between500 μs and 5 ms relative to the duration of the complete cycle. TheDC/DC converter 3 is deactivated throughout this phase, and the DC/ACconverter 6 then drains significant energy from the capacitor 4. Thisleads to a progressive drop in the intermediate voltage 5 which becomessignificant. This intermediate voltage 5 that has become very low mayrequire significant time to be recharged, the time not necessarily beingavailable before the start of the next injection phase.

A second drawback is associated with the operating principle of theDC/AC converter 6. If the intermediate voltage 5 that powers the DC/ACconverter 6 varies too much, the latter loses its accuracy, and nolonger guarantees the characteristics of the final signal 9.

It would therefore appear desirable to mitigate these drawbacks bymaintaining an intermediate voltage 5 that is as stable as possible inorder to be able to modulate the amplitude of the final voltage pulsetrain 9 at the output of the DC/AC converter 6 as accurately aspossible.

The present invention remedies these various drawbacks, and its subjectis a voltage pulse train generator comprising a voltage source supplyingan initial DC voltage, a DC/DC converter powered by the initial voltageand able to charge a capacitor to an intermediate DC voltage greaterthan the initial voltage, a DC/AC converter operating in switched modeby alternating active phases and inactive phases, able to transform theintermediate voltage obtained from the capacitor into a final voltagepulse train, and a control unit for driving the converters. According toone feature of the invention, the DC/DC converter is able to operate, inorder to charge the capacitor, at the same time as the DC/AC converter,at most during the inactive switching phases of the DC/AC converter.

Another subject of the invention is an ultrasound piezoelectric injectorcontrol device, comprising at least one voltage pulse train generatoraccording to one of the preceding embodiments.

Another subject of the invention is a method of using such an ultrasoundpiezoelectric injector control device, comprising a holding step, in theinjection phase, during which the two converters operate in phaseopposition, one of the converters being in the active phase at mostduring the inactive phase of the other converter, and vice versa, inorder to control an ultrasound piezoelectric injector.

One advantage of the device according to the invention is that it allowsthe inactive phases of the DC/AC converter to be used to operate theDC/DC converter and thus recharge the capacitor more regularly and for alonger time.

Because of the two-phase operation as described, a certain residualripple on the intermediate voltage may be observed. However, theamplitude of this ripple is directly linked to the value of thecapacitor and can advantageously be reduced by increasing the value ofsaid capacitor.

Another advantage of the device according to the invention is that itenables the DC/AC converter to instantaneously modulate the voltageacross the terminals of the injector because of a stable and regulatedintermediate voltage, and thus to produce enhanced driving quality andfinesse.

Another advantage of the device according to the invention is that thecapacitor is much less stressed. The lower the voltage fluctuationsacross the terminals of a capacitor, the less energy is needed to beable to recharge it to a desired voltage set point. Current inrush isalso lower. Controlling the intermediate voltage thus makes possiblesavings in dynamics and in recharging time of the capacitor. The powerabsorbed on the onboard network is thus less great. The DC/DC converterstage can be better dimensioned, with increased efficiency and a lowercost. It has already been stated that a high capacitor value isnecessary to reduce the residual ripple on the intermediate voltage.However, advantageously, the capacitor does not need to beoverdimensioned in terms of rms current.

Other features, details and advantages of the invention will become moreclearly apparent from the detailed description given below as anindication in relation to the drawings, in which:

FIG. 1 illustrates an ultrasound piezoelectric injector control devicein its usage context,

FIG. 2 illustrates a particular embodiment of such a control device,

FIG. 3 illustrates a timing diagram of use of an ultrasoundpiezoelectric injector control device,

FIG. 4 illustrates the same timing diagram for a particular embodiment.

FIG. 1, already described in relation to the prior art, shows, accordingto the invention, a control device 12 comprising a voltage pulse traingenerator 11 comprising a voltage source 1 supplying an initial DCvoltage 2, a DC/DC converter 3 powered by the initial voltage 2 and ableto charge a capacitor 4 to an intermediate DC voltage 5 greater than theinitial voltage 2, a DC/AC converter 6 operating in switched mode byalternating active phases 7 and inactive phases 8, able to transform theintermediate voltage 2 obtained from the capacitor 4 into a finalvoltage pulse train 9, and a control unit 10 for driving the converters3, 6. According to the invention, the DC/DC converter 3 is able tooperate, in order to charge the capacitor 4, at the same time as theDC/AC converter 6, at most during the inactive switching phases 8 of theDC/AC converter 6. In practice, the DC/AC converter 6 operatingaccording to the switching principle will exhibit a succession of activephases 7 and inactive phases 8. By this very principle, the DC/ACconverter 6 short circuits the capacitor 4 through an inductor, by meansof a transistor, during an active phase 7. However, during an inactivephase 8, this transistor is blocked. This blocking can be exploited torecharge said capacitor 4 by operating the DC/DC converter 3 during allor part, or for at most the full duration, of an inactive phase 8. Itwould be possible not to use some of the inactive phases 8.

A converter is said to be active when said converter drains energy fromits source. On the other hand, a converter is said to be passive orinactive when the stored energy is transferred down line.

FIG. 2, taken from FR 04/13277, illustrates a particular embodiment of acontrol device 12. The different stages of the pulse generator11/ultrasound piezoelectric injector control device 12 are shown fromleft to right. A battery E is a voltage source 1. The set of componentscomprising an inductor L1, a diode D and a switch ID1 coupled to a dioded1 constitutes, with a capacitor C, 4 at the output, the DC/DC converter3, also identified ε1. The set of components comprising an inductor Lrand a switch ID2 coupled to a diode d2 constitutes the DC/AC converter6, also identified ε2. These three sets constitute, with a control unit10 that is not represented, a voltage pulse train generator 11. Thefunctional switches ID1 and ID2 are advantageously transistors in orderto be able to be controlled. The function of the control unit 10 is tocontrol these transistors, this providing the switching and thereforeimplementing the DC/DC and DC/AC functions by respectively driving theswitches ID1 and ID2.

Said generator 11 is complemented with a third stage ε3 comprising aninjector selector 14, Si. The generator 11 and the selector assembly 14constitute a control device 12. Each selector Si is associated with arespective injector 13. Thus, the opening of all the selectors Si exceptthat of a given injector 13 causes the voltage pulse train 9 produced bythe generator 11 to be sent to said given injector 13, and controls theinjection.

The selector 14 also serves as a multiplexer in an assembly with agenerator 11 and a plurality of injectors 13. It is also possible toeliminate the selector 14 by using a generator 11 associated with eachinjector 13. However, such an arrangement is not economicallyadvantageous, and the multiplexed assembly is preferred. Such amultiplexed assembly does, however, have the drawback of reducing thetime available between two injections, all the more so as the number ofinjectors 13 increases. In order to recharge the capacitor 4, the DC/DCconverter 3 has, within the limits of the prior art, only the timebetween the end of an injection for an injector 13 and the start of theinjection of the next injector. Here too, the invention that makes itpossible to charge the capacitor 4 while the DC/AC converter 6 isoperating appears very advantageous.

Like the DC/AC converter 6, the DC/DC converter 3 can also operate inswitched mode. One example of such a converter is given in the assemblydescribed previously in conjunction with FIG. 2. The switching of theDC/DC converter 3, controlled by the control unit 10, is advantageouslyperformed at a switching frequency identical to that of the DC/ACconverter 6, the two converters operating in phase opposition, one ofthe converters (DC/DC, or DC/AC) being in the active phase 7 at mostduring the inactive phase 8 of the other converter (DC/AC, or DC/DC).Thus, the circuit of the DC/AC converter is never active at the sametime as the circuit of the DC/DC converter. According to the embodimentof FIG. 2, the switches ID1 and ID2 are never simultaneously closed. Theactive phases of each of the converters 3, 6 are interleaved.

With reference to FIG. 3, this describes an operating timing diagram ofthe generator 11/control device 12. The bottom portion of timing diagramshows, as a function of time, the behavior of the DC/AC converter 3 andthe top portion shows the behavior of the DC/AC converter 6. The diagramis periodic, the period being limited between two injection startinstants 19. The duration of the injection phase is typically between0.5 and 5 ms. At the injection start instant 19, an injection phase 18begins, ending at the injection end instant 20, and followed by arecharging phase 17.

During the injection phase 18, a holding phase 16 is applied. Duringthis holding phase 16, the two converters 3, 6 operate in phaseopposition, one of the converters 3, 6 being in active phase 7 at mostduring the inactive phase 8 of the other converter 6, 3, and vice versa.The DC/AC converter 6 operates in switched mode by alternating theactive phases 7 and the inactive phases 8 according to a predeterminedswitching frequency, for example equal to 45 kHz. The duty cycle is, forexample, 0.5. The DC/DC converter 3 exploits the inactive phases 8 ofthe DC/AC converter 6 in order to operate. In the case of a switchedmode DC/DC converter 3, its own active phases are included in theinactive phases 8 of the DC/AC converter 6. Thus, the intermediatevoltage 5, during the holding phase 16, exhibits ripple: it reducesduring an active phase 7 of the DC/AC converter 6 which discharges thecapacitor 4 and it increases during an inactive phase 8 of the DC/ACconverter 6, the DC/DC converter 3 charging said capacitor 4. Theintermediate voltage 5 thus remains substantially constant during theholding phase.

The holding phase 16 may be entirely concurrent with the injection phase18. Alternatively, for issues more particularly discussed in FR04/13277, in order to regulate transient phenomena, the holding phase 16may advantageously be preceded by an activation phase 15, at the startof the injection phase 18. During this activation phase 15, the DC/DCconverter 3 remains inactive, whereas the DC/AC converter 6 operates. Inthis phase, the intermediate voltage 5 decreases. This activation phase15 remains short in relation to the holding phase 16 and typically lastsbetween 20 and 200 μs.

After the injection phase 18, there is a recharging phase 17, duringwhich the DC/DC converter 3 operates in order to recharge the capacitor4, whereas the DC/AC converter 6 does not operate. This phase begins atthe start of injection 20 of an injector 13 and ends with the start ofinjection 19 of the next injector 13 in the injection sequence.

During the recharging phase 17, the intermediate voltage 5 increasesaccording to a profile 21 dependent on the characteristics of the DC/DCconverter 3 and its driving. Thus, in the case of a switched mode DC/DCconverter, the control unit can vary a number of characteristics of theDC/DC converter 3. This is done independently of the DC/AC converter 6which is inactive, and therefore independently of the resonancefrequency of the injectors 13. The control unit 10 thus determines aswitching frequency. A typical value is 100 kHz. In order to obtain theintermediate voltage value 5 necessary for the operation of the DC/ACconverter at the start of the next injection cycle, the control unit 10may vary the switching duty cycle, typically between 0 and 0.9. It canalso vary the operating time. Thus, if the capacitor 4 is sufficientlyrecharged before the next start of injection 19, the driving of theDC/DC converter may be stopped.

Referring to FIG. 4, this illustrates an advantageous embodiment. Thetiming diagram of FIG. 4 indicates, as a function of time, the relativeactivity phases of the DC/AC converter 6 and of the DC/DC converter 3.Thus, one of the converters 3, 6 is active in a phase 22 and inactive ina phase 23. The other converter 6, 3 is active 24, with, advantageously,an activity 24 positioned at the end of the inactivity phase 23 of thefirst converter 3, 6.

1-10. (canceled)
 11. A voltage pulse train generator comprising: avoltage source supplying an initial DC voltage; a DC/DC converterpowered by the initial voltage and configured to charge a capacitor toan intermediate DC voltage greater than the initial voltage; a DC/ACconverter operating in a switched mode by alternating active phases andinactive phases, configured to transform the intermediate voltageobtained from the capacitor into a final voltage pulse train; and acontrol unit that drives the converters, wherein the DC/DC converter isconfigured to operate, to charge the capacitor, at a same time as theDC/AC converter, at most during the inactive switching phases of theDC/AC converter.
 12. The generator as claimed in claim 11, in which theDC/DC converter operates in the switched mode, with a switchingfrequency identical to that of the DC/AC converter, the two convertersoperating in phase opposition, one of the converters being in the activephase at most during the inactive phase of the other converter, and viceversa.
 13. An ultrasound piezoelectric injector control device,comprising: at least one voltage pulse train generator as claimed inclaim 11, a final voltage pulse train controlling at least oneultrasound piezoelectric injector during the injection phase.
 14. Thedevice as claimed in claim 13, further comprising an injector selectorthat sends the final voltage pulse train obtained from the voltage pulsetrain generator to a predetermined injector of a plurality of injectors.15. The device as claimed in claim 13, further comprising a voltagepulse train generator associated with each injector.
 16. A method ofusing an ultrasound piezoelectric injector control device as claimed inclaim 13, comprising a holding, in the injection phase, during which thetwo converters operate in phase opposition, one of the converters beingin the active phase at most during the inactive phase of the otherconverter, and vice versa, to control an ultrasound piezoelectricinjector.
 17. The method as claimed in claim 16, further comprising,before the holding, an activation, at a start of the injection phase,during which the DC/DC converter does not operate, while the DC/ACconverter operates to activate an ultrasound piezoelectric injector. 18.The method as claimed in claim 16, further comprising, after theholding, recharging the capacitor, outside the injection phase, duringwhich the DC/DC converter operates to recharge the capacitor, while theDC/AC converter does not operate.
 19. The method as claimed in claim 18,in which the switching of the DC/DC converter exhibits a duty cycleand/or an operating time that are variable to vary recharging as afunction of the desired intermediate voltage and of the time availablebefore the next injection phase.
 20. The method as claimed in claim 16,in which one of the converters is in active phase at the end of theinactive phase of the other converter, and vice versa.